Implement automatic spell-check for docs (#688)

* [WIP] Implement automatic spell-check for docs

fixes #663

The only thing unclear to me so far is how the output of `make spelling`
should be checked - is there a possibility to check for a file (i.e.
`_build/spelling/output.txt`) to be empty, and report a failed build
otherwise?

* fix typo in requirements.txt

* add enchant library

* travis should report errors, order spelling wordlist

* change travis.yml to easier troubleshoot build issues

* fixed more typos, added more words

* add more words, fix more typos

* added more words

* added more words

* revert changes to .travis.yml

doc/spelling_wordlist.txt

@@ -0,0 +1,118 @@
+addon
+addons
+api
+auth
+autobuild
+backend
+backends
+banktransfer
+boolean
+booleans
+cancelled
+casted
+checkbox
+checkin
+checkins
+checksum
+config
+contenttypes
+contextmanager
+cron
+cronjob
+debian
+deduplication
+discoverable
+django
+dockerfile
+durations
+eu
+filename
+filesystem
+fontawesome
+frontend
+frontpage
+gettext
+gunicorn
+hardcoded
+invalidations
+iterable
+libsass
+linters
+memcached
+metadata
+middleware
+mixin
+mixins
+multi
+multidomain
+namespace
+namespaced
+namespaces
+namespacing
+natively
+nginx
+ons
+optimizations
+param
+percental
+positionid
+pre
+prepend
+prepended
+prepending
+preprocessor
+presale
+pretix
+pretixdroid
+pretixpresale
+prometheus
+proxied
+proxying
+queryset
+redemptions
+redis
+refactored
+regex
+renderer
+renderers
+reportlab
+screenshot
+serializers
+serializers
+sexualized
+startup
+stdout
+stylesheet
+subdirectories
+subdirectory
+subdomain
+subdomains
+subevent
+subevents
+submodule
+subpath
+systemd
+testutils
+timestamp
+un
+unconfigured
+unix
+unprefixed
+untrusted
+username
+url
+viewset
+viewsets
+webhook
+webhooks
+webserver
+webservice
+workflow
+zipcode
+Datetime
+Embeddable
+Hierarkey
+OAuth
+SSL
+Uptime
+Yay